Neoplastic transformation produces many changes in cell physiology, some of which can be studied using morphologic techniques. We have used morphologic methods to study three general areas: 1) We have employed light microscopic immunocytochemistry to evaluate and select monoclonal antibodies for use as immunotoxin reagents for cancer therapy. A large scale screening chamber has been devised for use with immunofluorescence as a primary selection for surface-reactive monoclonal antibodies to human ovarian cancer cells. Hybridoma clones selected in this way were further evaluated by screening against normal human tissues and ovarian tumors using peroxidase immunocytochemistry in cryostat sections. Using these methods we have found two hybridomas highly reactive with human ovarian tumors: OVB1 and OVB3. We have characterized the tissue distribution and biochemical properties of these antigens in cultured cells, in tumors and in normal tissues. 2) We have studied the phenomenon of multidrug-resistance (MDR) in cultured cells, and have used a monoclonal antibody (MRK-16) to the human mdrl gene product (P170) as a probe to demonstrate P170 on the surface and in the Golgi stacks of drug-resistant KB cells in culture. We also showed the absence of P170 from coated pits, suggesting that this protein is immobilized in the plasma membrane. We have also used this antibody to examine the distribution of P170 in normal human tissues, and we have evaluated the amounts of P170 in progressively resistant derivatives of KB cells using immunofluorescence. 3) In other studies, we have localized p55, a major thyroid hormone binding protein found in the endoplasmic reticulum and nuclear envelope of cultured cells in normal tissues using immunoperoxidase histochemistry. This protein was found in highest amounts in cells that have abundant endoplasmic reticulum. Other cytochemical studies demonstrated a role for clathrin-coated pits in the loss of ADH-responsive water transport in isolated perfused rabbit cortical collecting ducts. We also demonstrated the localization of carbohydrate sites reactive with wheat germ lectin on nuclear pores in isolated nuclei.